Electrolytic capacitors that make use of a valve metal, such as tantalum (Ta), niobium (Nb), and aluminum (Al), as an electrode have been known. The electrolytic capacitors have been widely used because they are small in size yet achieve a large capacity in comparison with other types of capacitors. Among the electrolytic capacitors, wound-type solid electrolytic capacitors that employ, as the electrolyte, polypyrrole-based, polythiophene-based, polyfuran-based, or polyaniline-based conductive polymer, or a TCNQ (7,7,8,8,-tetracyanoquinodimethane) complex salt, for example, have drawn attention.
It has been required to further increase the capacity of the solid electrolytic capacitors without increasing the size. In order to achieve this, a method of increasing the capacitance by forming a coating film made of a metal nitride such as titanium nitride (TiN) on the surface of the cathode has been proposed.
Here, a wound capacitor element in a conventional wound-type solid electrolytic capacitor will be described below. FIG. 5 is a schematic perspective view illustrating one example of the structure of the wound capacitor element in a conventional wound-type aluminum solid electrolytic capacitor. FIG. 5 shows the wound capacitor element whose trailing end is unwound, for clearly illustrating the stack structure in the capacitor element.
The capacitor element 1 comprises: an aluminum foil serving as an anode 2, which has been subjected to an etching process (electrolytic polishing process) and a formation process; an aluminum foil serving as a cathode 3, which has been subjected to an etching process; a TiN deposition film serving as a hard coating film 8; separator papers 4a and 4b for preventing the aluminum foil 2 and the aluminum foil 3 from making contact with each other; and polythiophene-based conductive polymer layers (not shown) each serving as a solid electrolyte layer, which are formed between the aluminum foil 2 and the separator paper 4b and between the TiN deposition film and the separator paper 4b. The capacitor element 1 further comprises a winding fastening tape 5 for retaining the wound state, an anode lead tab 6a and a cathode lead tab 6b respectively connected to the anode 2 and the cathode 3, and an anode lead wire 7a and a cathode lead wire 7b respectively connected to the anode lead tab 6a and the cathode lead tab 6b.
The capacitor element 1 is fabricated through the following processes. An aluminum foil is subjected to an etching process (electrolytic polishing process) and a formation process to prepare the anode 2. After preparing the anode 2, an anode lead tab 6a is provided on the anode 2. Also, an aluminum foil is subjected to an etching process to prepare the cathode 3. After preparing the cathode 3, a TiN deposition film, serving as the hard coating film 8, is formed on the surface of the cathode 3. Thereafter, the cathode lead tab 6b is provided on the cathode 3. The separator paper 4a, the anode 2, and the cathode 3 on which the separator paper 4b and the hard coating film 8 have been formed, are successively stacked, and thereafter, the stacked material is wound in a cylindrical form and fastened with the winding fastening tape 5. Subsequently, the wound stacked material is subjected to an edge formation and a heat treatment at 280° C. Then, the wound stacked material is immersed into a preparation solution in which 3,4-ethylenedioxythiophene has been mixed with an alcohol solution containing 40 mass % to 60 mass % of ferric p-toluenesulfonate (an oxidizing agent solution). Next, the wound stacked material is heated to thermal polymerize the 3,4-ethylenedioxythiophene. Thereby, the polythiophene-based conductive polymer layers (the solid electrolyte layers) are formed between the aluminum foil 2 and the separator paper 4b and between the hard coating film 8 and the separator paper 4b. Lastly, the anode lead wire 7a and the cathode lead wire 7b are connected respectively to the anode lead tab 6a and the cathode lead tab 6b. 
The above-described capacitor element 1 has the following problems. Since the cathode 3 in which the titanium nitride deposition film has been formed as the hard coating film 8 undergoes stress, such as a stretching force or a twisting force, when winding the stacked material to form the capacitor element 1, cracks develop in the titanium nitride deposition film, resulting in an increase in leakage current of the capacitor element 1. In addition, when immersing the stacked material into the preparation solution to form the polythiophene-based conductive polymer layers as the solid electrolyte layers, the preparation solution corrodes the titanium nitride deposition film, resulting in an increase in leakage current of the capacitor element 1.
As a method for resolving the foregoing problems, there has been a proposal (see Patent Reference 1 below) to prevent the increase of the leakage current in the capacitor element resulting from the corrosion and cracks of the hard coating film by forming, in place of the hard coating film composed of a single metal compound, such as the TiN deposition film, a hard coating film composed of a composite metal compound such as titanium aluminum nitride (TiAlN) on the surface of the cathode.
Patent Reference 1: Japanese Published Unexamined Patent Application No. 2004-221512